lavf/dump: dump the vbv_delay with N/A instead of 18446744073709551615

How to check it:
./ffmpeg -f lavfi -i testsrc  -c:v mpeg2video  -f null  -
master:
  Side data:
        cpb: bitrate max/min/avg: 0/0/200000 buffer size: 0 vbv_delay:
        18446744073709551615
patch applied:
Side data:
      cpb: bitrate max/min/avg: 0/0/200000 buffer size: 0 vbv_delay: N/A

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Limin Wang <lance.lmwang@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/dump.c

@@ -321,13 +321,16 @@ static void dump_cpb(void *ctx, AVPacketSideData *sd)
 
     av_log(ctx, AV_LOG_INFO,
 #if FF_API_UNSANITIZED_BITRATES
-           "bitrate max/min/avg: %d/%d/%d buffer size: %d vbv_delay: %"PRIu64,
+           "bitrate max/min/avg: %d/%d/%d buffer size: %d ",
 #else
-           "bitrate max/min/avg: %"PRId64"/%"PRId64"/%"PRId64" buffer size: %d vbv_delay: %"PRIu64,
+           "bitrate max/min/avg: %"PRId64"/%"PRId64"/%"PRId64" buffer size: %d ",
 #endif
            cpb->max_bitrate, cpb->min_bitrate, cpb->avg_bitrate,
-           cpb->buffer_size,
-           cpb->vbv_delay);
+           cpb->buffer_size);
+    if (cpb->vbv_delay == UINT64_MAX)
+        av_log(ctx, AV_LOG_INFO, "vbv_delay: N/A");
+    else
+        av_log(ctx, AV_LOG_INFO, "vbv_delay: %"PRIu64"", cpb->vbv_delay);
 }
 
 static void dump_mastering_display_metadata(void *ctx, AVPacketSideData* sd) {